This invention relates to a color measuring apparatus and method for measuring color of a measurement object having an angular metamerism, such as metallic coating, in which aluminum flakes or mica flakes in a coating.
There have been known color measuring apparatuses for a metallic coating. A color measuring apparatus disclosed in Japanese Examined Patent Publication No. 63(SHO)-41019 measures color of a coating as follows: Illumination light is projected from a single illumination light source onto a coating as a measurement object. Among the light rays reflected by the coating, reflected light rays which are reflecting in a plurality of angular directions different from a direction of regular reflection are received by a plurality of light receiving elements. Color of the coating is measured based on signals outputted from the respective light receiving elements. This color measuring apparatus is of the unidirectional illumination/radial light reception type.
A color measuring apparatus disclosed in Japanese Examined Patent Publication No. 4(HEI)-29019 measures color of a coating as follows: Illumination light rays are intermittently projected at angles different from each other to a coating as a measurement object. Light rays reflected by the coating are successively received by a single light receiving element, and color of the coating is measured based on signals successively outputted from the light receiving element. This color measuring apparatus is of the radial illumination/unidirectional light reception type.
Further, a color measuring apparatus disclosed in Japanese Examined Patent Publication No. 5(HEI)-80614 measures color of a coating as follows: An illumination light ray is projected to a measurement object in one direction. A reflected light ray which is inclined by a specified angle from a direction of regular reflection is received by a light receiving element. Color of the coating is measured based on an output signal of this light receiving element. This color measuring apparatus is of the unidirectional illumination/unidirectional light reception type.
In the color measuring apparatus of the unidirectional illumination/unidirectional light reception type, if the metallic coating is illuminated in one direction, there arises a major problem of considerable angle dependency of the reflected light in the case that light is not reflected in a uniform direction in a range near the direction of regular reflection due to the orientation of bright material such as aluminum flakes or mica flakes, in the case that the surface of the measurement object is curved, in the case that the color measuring apparatus is slightly inclined, or other case.
The color measuring apparatus of the unidirectional illumination/radial light reception type is free from the above problem because light rays reflected in different directions are measured although the measurement object is illuminated in one direction, and color of the measurement object is measured based on the output signals of the light receiving elements having received the respective reflected light rays. The same can be said of the color measuring apparatus of the radial illumination/unidirectional light reception type.
In the conventional color measuring apparatuses of the unidirectional illumination/radial light reception type, light receiving elements are also arranged in a range near the direction of regular reflection (e.g., directions at 25.degree. or 45.degree. from the direction of regular reflection) and in a range relatively distant from the direction of regular reflection (e.g., 75.degree. from the direction of regular reflection), so as to enable the radial light reception. This makes the construction of the apparatus complicated, resulting in a larger size and an increased production cost.
Further, in the color measurement, it is essential to receive reflected light rays at angles larger than 90.degree. from the direction of regular reflection of the illumination light by the light receiving element and to measure color based on the output signals of the light receiving elements to evaluate the color performance in the thickness of a coating. This measurement is hereinafter called "color-in-thickness measurement." However, the conventional color measuring apparatuses are incapable of performing the color-in-thickness measurement due to its construction.